The invention relates to a glass composition for use in electric lamps.
The invention further relates to a stem for an electric lamp manufactured from such a glass composition.
The invention also relates to a fluorescent lamp comprising a vacuum-tight glass lamp envelope manufactured from such a glass composition.
In electric lamps, such as incandescent lamps anti fluorescent lamps, the lamp envelope generally consists of a cheap glass of the soda-lime type. However, the electric resistance of soda-lime glass is insufficient for use in the part of the lamp where the current supply conductors enter tile lamp envelope, so that leakage currents may occur in said part. Said part of the lamp is termed "stem" and comprises a glass flare, a glass exhaust tube, a glass support tool, current supply conductors, support wires and a coiled filament. Usually lead-containing glass comprising, for example, 20% by weight of PbO is used for making steins. PbO causes an increase of tile electric resistance of glass and it softens glass, which has a favorable effect on tile workability of glass.
A disadvantage of the use of PbO is its toxicity. In the preparation of lead glass, PbO is released into the atmosphere by dusting and evaporation, which is harmful to the environment and the operators. Also when lead glass is subjected to a hot working operation such as bending, moulding and fusing, PbO is released. Consequently, in order to avoid exposure to PbO the working environment has to be adapted drastically. Another disadvantage of PbO is the high cost of the raw material. A further disadvantage of PbO is the reduced light output ill compact fluorescent lamps, which is caused by evaporation followed by condensation of PbO on the fluorescent powder during hot working tile lead-containing tube glass and/or fusing the lead-containing stems. Therefore, the search for a low lead or rather lead-free glass which can be used for the manufacture of stems and which nevertheless has the desired physical properties as regards meltability, softening, expansion, electric resistance, transparency, strength and chemical resistance, has been going on for a considerable length of time.
U.S. Pat. No. 3,723,790 discloses low lead and lead-free glasses for use in electric lamps. The glass compositions described therein contain 0-8% by weight of PbO. However, the known lead-free glass compositions have a number of disadvantages. The known lead-free glass has too high a liquidus temperature (T.sub.liq) of 888.degree. C. The liquidus temperature is the temperature above which the glass no longer crystallizes. The lower this temperature, the smaller the risk that the glass crystallizes during the moulding process, for example the Vello process of drawing glass tubing. To lower the liquidus temperature to 837.degree. C., 1.2% by weight of B.sub.2 O.sub.3 is added to the known lead-free glass. A disadvantage of tile use of B.sub.2 O.sub.3 is its high price and the agressiveness relative to the refractory material of the glass furnace. Other disadvantages of the known lead-free glass are the high content of Li.sub.2 O (&gt;1.5% by weight) and K.sub.2 O (&gt;9% by weight), as a result of which the raw materials are expensive, and the high content of BaO (&gt;12% by weight) which leads to a substantial increase of the crystallization tendency of the glass. To the known lead-free glasses is also added an antimony-containing or arsenic-containing compound as the refining agent. However, these substances are very toxic and remain in the glass predominantly as Sb.sub.2 O.sub.3 or As.sub.2 O.sub.3, respectively.